Human attenuation correction in a positron emission tomography session can be carried out on the basis of magnetic resonance data. In magnetic resonance systems the volume that can be used for acquired magnetic resonance tomography images within the tomography apparatus is limited in all three spatial directions due to physical and technical constraints, such as a limited homogeneity of the magnetic field, for example, and a nonlinearity of the gradient field. For this reason a usable acquisition volume for magnetic resonance tomography images, what is referred to as a usable field of view (FoV), is restricted to a volume in which the aforementioned physical properties lie within predefined tolerance ranges and consequently true-to-the-original imaging of the object to be examined is possible using conventional magnetic resonance measurement sequences.
Strong distortions of the measurement object occur outside of this usable field of view and true-to-the-original imaging cannot be guaranteed using conventional measurement sequences. What is meant by distortion in this context is that a signal value of the examination object at a predefined location appears at a different location in the image of the examination object determined from the acquired magnetic resonance data.
The coordinates of the predefined location are also referred to as the actual position and the coordinates in the image determined from the acquired magnetic resonance data are also referred to as the distorted position. Distortions which cannot be compensated by subsequent rectification of the image of the examination object can occur in particular outside the usable field of view, because for example a plurality of adjacent actual positions can be mapped to one or more distorted positions that are located close to one another. The thus delimited field of view is considerably smaller, in particular in the x- and y-direction, i.e. perpendicularly to a longitudinal axis of a tunnel of the magnetic resonance system, than the volume delimited by the tunnel of the magnetic resonance system. In conventional magnetic resonance systems the tunnel has for example a diameter in the 60-70 cm range, whereas the diameter of the normally usable field of view in which the aforementioned physical conditions lie within the tolerance ranges is approximately 10 cm smaller, in other words in a range of 50-60 cm.
In an MR/PET system in which the human attenuation correction for the positron emission tomography session is to be carried out on the basis of the magnetic resonance data, an image of the examination object that is true to the original and precise in terms of location is also necessary in a peripheral region between the above-described normally usable field of view and the inner wall of the tunnel, since for example the arms of a patient may also be arranged in said peripheral region, the position of the arms being required for determining the attenuation correction for the acquisition of the PET image. Even small distortions of the underlying magnetic resonance data can greatly affect the attenuation correction factors, because the attenuation correction factors are an exponential function of the attenuation p, the correction of the PET scattering, called “scatter correction”, requires a distortion-free attenuation correction map, and the so-called “scatter scaling” requires a precise specification of the contours of the examination object.
According to the prior art, as known for example from the publication titled “Tissue classification as a potential approach for attenuation correction in whole-body PET/MRI: evaluation with PET/CT data” by Martinez-Möller A, Souvatzoglou M, Delso G, et al., from J Nucl Med. 2009; 50:520-526, an attenuation correction in the torso can be determined by way of a 2-point DIXON method and a segmentation into four classes (soft tissue, fat, lung, background).
It is also known for example from the publication titled “Completion of a Truncated Attenuation Image from the Attenuated PET Emission Data” by Nuyts J, Michel C, Fenchel M, Bal G, Watson G, from IEEE Nucl Sci Symp Conf Record 2010, that missing portions in an attenuation correction map can be estimated retrospectively from the PET data by way of a maximum-likelihood a-posteriori algorithm. However, a disadvantageous aspect with this method is the great amount of time required for performing the calculation as well as a limitation to specific PET tracers having a sufficiently high uptake in the arms and skin. Furthermore, there can be difficulties with certain PET applications (for example dynamics), since the profile of the PET tracer uptake can change over time. What's more, the accuracy of the position determination is dependent on the quality of the PET statistics, since the actual position of the arms for example is not determined by MR-based measurement, but is calculated from the PET projection data.
Furthermore, a method for determining a position of a subregion of an examination object in a magnetic resonance system is disclosed by the patent application with the application number DE 10 2010 006 431 A1 of the same inventor. The subregion of the examination object is arranged at the edge of the field of view of the magnetic resonance system. With the method, at least one slice position for a magnetic resonance image in which the B0 field at the edge of the magnetic resonance image fulfills a predetermined homogeneity criterion is determined automatically. Furthermore, a magnetic resonance image including the subregion at the edge of the field of view is acquired in the specific slice position. The location of the subregion of the examination object is determined automatically on the basis of the location of the subregion in the acquired magnetic resonance image. A disadvantageous aspect with this method is the additional time required for the additional magnetic resonance measurement.